Player has to fall into the void to generate the world

When I open a new world, I am in the sky; everything is blue. Then I fall down into the void, out of the world, but I take no damage. Once, I have spawned, I cannot move and I am stuck in a block. Only until I destroy the block I will be released.

My .minecraft file is modded, but only with Optifine. I'm sure this mod is accepted.